flang is missing OpenMP omp_lib.mod

[scivision]

brew gist-logs <formula> link OR brew config AND brew doctor output

HOMEBREW_VERSION: 4.4.12
ORIGIN: https://github.com/Homebrew/brew
HEAD: 3001afe88112c13b5aafdd92e097680897060881
Last commit: 3 days ago
Branch: stable
Core tap JSON: 18 Dec 14:11 UTC
Core cask tap JSON: 18 Dec 14:11 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.CjoOMKE3LL/org.xquartz:0
HOMEBREW_EDITOR: nano
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 3.3.6 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.6/bin/ruby
CPU: octa-core 64-bit arm_blizzard_avalanche
Clang: 16.0.0 build 1600
Git: 2.47.1 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.2-arm64
CLT: 16.2.0.0.1.1733547573
Xcode: 16.2
Rosetta 2: false

Your system is ready to brew.

### Verification

- [X] My `brew doctor` output says `Your system is ready to brew.` and am still able to reproduce my issue.
- [X] I ran `brew update` and am still able to reproduce my issue.
- [X] I have resolved all warnings from `brew doctor` and that did not fix my problem.
- [X] I searched for recent similar issues at https://github.com/Homebrew/homebrew-core/issues?q=is%3Aissue and found no duplicates.

### What were you trying to do (and why)?

Compile a trivial OpenMP program.

```fortran
program a

use omp_lib, only: omp_get_num_procs

print *, omp_get_num_procs()

end program

What happened (include all command output)?

error: Cannot read module file for module 'omp_lib': Source file 'omp_lib.mod' was not found

What did you expect to happen?

Expected file "omp_lib.mod" to be present under "/opt/homebrew/Cellar/flang/19.1.5/include/flang/" so that OpenMP Fortran program builds.

It could be that OpenMP isn't built for LLVM in the flang formula. Reference: llvm/llvm-project#90543

Step-by-step reproduction instructions (by running brew commands)

brew install flang

flang-new -fopenmp omp.f90 

Where omp.f90 is the source code above.

An equivalent C++ program builds successfully by:

brew install clang libomp

r=$(brew --prefix)/opt

$r/llvm/bin/clang++ -I$r/libomp/include omp.cpp -L$r/libomp/lib -lomp

#include <omp.h>
#include <iostream>
int main(){
std::cout << omp_get_num_procs() << "\n";
return 0;
}```

[carlocab]

It could be that OpenMP isn't built for LLVM in the flang formula. Reference: llvm/llvm-project#90543

This is correct. Pull requests to fix this welcome.